314 TEMPERA TUBE. 



* 



49 1 . Practical Results. The ascension of ' ' fire-balloons " 

 and the draft of chimneys are due to the expansion of gases by heat. 

 When the air in the chimney of a stove or lamp is heated, it is ren- 

 dered lighter than the same bulk of surrounding air, and, therefore, 

 rises. The cooler air comes in to take its place and thus feeds the com- 

 bustion. Sometimes when a fire is first lighted, the chimney is so 

 cold that the current is not quickly established and the smoke 

 escapes into the room. But in a little while the air column rises 

 and the usual action takes place. By the aid of a good thermometer 

 it may be shown that the air near the ceiling of a room is warmer 

 than the air near the floor. When the door of a warmed room is 

 left slightly ajar, there will be an inward current near the floor and 

 an outward current near the top of the door. These currents may 

 be shown by holding a lighted candle at these places. Artificial 

 ventilation depends upon the same principles. 



492. Rate of Gaseous Expansion. The rate 

 of expansion is practically the same for all gases, viz., 

 0.00336 or ^ of the volume at 0., for each centigrade 

 degree that the temperature is raised above the freezing 

 point. In other words, a liter of air at C., expands to 



11 + .00336 I at 1 C., 

 11+ (.00338 x 2) I at 2 C. 



Of course, if we use Fahrenheit degrees the expansion 

 will be only as great, or about ?fa. A litre of gas at 32 F. 

 expands to Ijfo I at 33 F. ; to ffl I at 39 F., etc. 



11+ (.00336 x 3)?. at 3 C., 

 I at 4 C. 



493. Absolute Zero of Temperature. The 



temperature at which the molecular motions con- 

 stituting heat wholly cease is called the absolute 

 zero. It has never been reached, and has been only ap- 

 proximately determined, but it is convenient as an ideal 

 starting-point. The zero point of the thermometers does 

 not indicate the total absence of heat. A Fahrenheit 

 thermometer, therefore, does not indicate that boiling 

 water is 212 times as hot as ice at 1 F. ; a centigrade 



